EVOLUTION OF A MESOSCALE UPPER-TROPOSPHERIC VORTICITY MAXIMUM AND COMMA CLOUD FROM A CLOUD-FREE 2-DIMENSIONAL POTENTIAL VORTICITY ANOMALY

A diagnostic case study is presented in which output from the limited- area forecast version of the Meteorological Office operationaL Unified Model was used together with water vapour and cloud imagery from the Meteosat satellite. The case chosen for study was an archetypally simp le example in which a long, narrow strip of air with high potential vo rticity (PV) in the upper troposphere-strictly a tropopause depression -became unstable and rolled up into fairly long-lived mesoscale vortic es. For much of the time the vortices were cloud-free and they were dy namically pure examples of very small upper-air PV anomalies moving re lative to the strata beneath. As such they provided a good test of the oretical deductions from the PV invertability principle and of the per formance of the dynamical components of the forecast model. The evolut ion of the mesoscale vortices as revealed in the satellite imagery was well handled by the Unified Model, indicating that the small scales w ere reproduced well by the model dynamics even though they were not fu lly defined by the routine observations fed into the model. As the vor tices began to form, the stretching deformation along the axis of the initial PV strip was very small, and the 900 km spacing of the vortice s was broadly consistent with theoretical expectations for the wavelen gth of instabilities growing on such a PV strip. A detailed analysis o f one of the mesoscale vortices throughout its 3-day lifetime showed t hat its maximum potential vorticity and absolute vorticity decreased w ith time as it travelled within the circulation of a large-scale antic yclonic gyre. By the end of the second day, when cloud first formed, t he vortex had become a weak upper tropospheric shear line, 500 km long , sandwiched between a pair of anticyclonic mesoscale vortices 900 km apart. The Meteosat imagery revealed a characteristic two-component st ructure both in the water-vapour channel and, when the cloud eventuall y formed, in the infrared channel too. This structure, which resembled that seen on other occasions, is explained in terms of a particular c ombination of vertical motion and advection in the region of sharp hum idity gradients alongside dry stratospheric air that had intruded into the upper troposphere.